Our house, bought last fall, came with a water feature in the back yard. It has a waterfall at the top, and the water cascades through several small ponds on the way to the bottom, where the biggest pond is. The system was somewhat neglected, probably because the owners were snowbirds who were not around the place much. We were hoping to get tips on caring for the pond from them, but we never met them. We have gleaned some information from the internet, but this problem has us stumped:

Every time it rains, the bottom pond loses water, sometimes to the point where the pump is partly exposed. No clue where the water goes - it may be into our septic tank for all we know. We refill it, and everything is fine until the next rainstorm. When we cleaned the pond this past spring, we noticed the bottom of the big pond is not solid plastic. It has a grid of small holes in it, that we assume feed into a bottom drain.

What causes this? Could there be a relay in the system that causes the pond to drain whenever it rains? Why only when it rains? It doesn't happen at any other time.

The picture is of the bottom pond. I'm not even going to address the problem with the extension cords everywhere (with no GFIs until we had them installed). We turned off the pump because the water is so low. When it stops raining we will refill and turn the pump back on. We know next to nothing about ponds and how they (should) work. This is driving us crazy!